If, like me, you have a real problem with butchers string creating a good/tight knot that won't give, especially when using the stripy sort when used with heavy salami my guess is because it is made from rayon/synthetic, or at least has a synthetic coloured thread that doesn't tighten in the same way that natural fibres do.

It seems that traditional baker's twine (as opposed to butcher's twine)is still easily available as a 100% cotton option and this could be better suited for salami (although as it probably doesn't conform to the various food law it would probably not be lawful to use commercially). Many different suppliers on the web, but virtually all seem to be made by James Lever:

This is a very useful post. Oddly enough, the OP was made more or less to the day that I was in the south of France and finding this is a little hardware store:

Unbleached twine at €3.90 for 160m I though was a very good deal (hence I bought three).

Like Swing Swang, I was also looking for natural twine for my salami. Not because of problems with the stripey butcher's twine (I find that mine makes a good knot, especially when it is slightly damp) but because wanted a more artisanal look.

Here it is in use:

Now I just need to learn how to tie 'em up nice, like this (also found this June in the South of France):

(ignoring the fact that the twine came off at one end as the sausage shrunk)

To keep your knots tight, do a double loop, then a single on the knot, the double loop will help keep the line taught while you tie the second loop of the knot, I also keep a finger on the first double loop while looping the second knot. Also help if the cotton cord is wet.

Also twisting the string around a couple of dowels if you need to pull the knot really tight helps you get a better grip without blistering/cutting into the last joint of your little fingers if you are doing a lot.